Solve for $a$ : $-10 = 28 + a$
Subtract $28$ from both sides: $ \begin{eqnarray} \\ \hphantom{-10 {- 28}} &\hphantom{=}& \hphantom{a} \\ -10 &=& 28 + a \\ \\ {-28} && {-28} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ -10 {- 28} &=& a \\ \end{eqnarray} $ Simplify. $ a = -38$